1. Field of the Invention
The present invention relates to an image processing method and apparatus for sharpening (edge sharpening) photographic image data obtained by digitizing, by using a film scanner, a photographic image formed on a photographic film or photographic image data obtained by directly digitizing a photographic image by a digital photographic device such as a digital camera. The method effects the sharpening by utilizing a sharpening filter technique capable of sharpening intensity adjustment. The invention relates also to a computer-executable program for implementing the method as well as an apparatus for implementing the method.
2. Description of the Related Art
As is known, in a system for effecting an image processing on photographic image data inputted thereto and outputting the processed image through a photographic printer, sharpness deterioration occurs in its output image. Therefore, the system effects an image sharpening operation on the inputted photographic image data by using a Laplacian filter or un-sharpening mask. However, while such image sharpening operation, if effected, indeed improves the (edge) sharpness of the photographic image, this operation also results in undesired enhancement of noise components included in the photographic image, thus inviting image quality deterioration. Especially, such noise enhancement effect resulting from the image sharpening operation will manifest itself more conspicuously in an image area such as of sky, human skin, etc. which extends uniformly without fine line patterns.
One conventional solution to this problem is known from e.g. Japanese Patent Application “Kokai” No. 2000-278532 (Document 1) (paragraphs [0007] through [0008], FIG. 1). An image processing apparatus disclosed in this document includes a line image detecting means for determining whether a predetermined pixel in inputted photographic image data constitutes a line image or not and a sharpening means for effecting a sharpening operation only on image data corresponding to the line image, based on the detection by the line image detecting means, so that only such image data corresponding to the line image may be enhanced. With this, even when e.g. unnecessary data such as those relating to film grains, noise, etc. are present over the entire photographic image data, undesired enhancement of the unnecessary data in the whole image can be avoided. As a result, there is obtained an image with appropriate sharpening without visible roughness in the sharpened image. Yet, even with this image sharpening technique, noise can be erroneously detected as a line image in the case of image data having significant noise amplitude variation, such as image data based on a photographic image of defective exposure. Hence, as such pixels also are subjected to the sharpening operation, the noise will be enhanced in the outputted photographic image.
Another solution is known from e.g. Japanese Patent Application “Kokai” No. 2003-101784 (paragraphs [0006] through [0007], FIG. 1). According to this document, an image processing apparatus capable of effecting a sharpening operation on image data operates as follows. First, the apparatus calculates, from the inputted image data, a local standard deviation indicative of pixel value variation within a predetermined area and then normalizes this local standard deviation, thereby to obtain an index value indicative of a degree of pixel value variation. Then, the apparatus calculates a sharpness enhancement coefficient indicative of intensity of sharpness enhancement, whose value monotonously varies in association with increase in the index value, so that the apparatus effects the sharpening operation on the image data by using the sharpness enhancement coefficient. In the case of this image sharpening technique, in an attempt to solve the above-described problem, the sharpening operation is effected selectively in such a manner that photographic image data based on a subject with less conspicuous noise component is subjected to a sufficient and necessary degree of sharpness enhancement whereas photographic image based on a subject with conspicuous noise component is subjected to a lesser degree of sharpness enhancement. This technique employs a statistical approach for entire image. Hence, while the technique can achieve a certain average or overall effect, the technique can hardly achieve any significant image quality improvement within a specified area in the image, such as an area of human skin, sky, etc. which area has greater visual importance in the photography.